This invention relates to expandable medical implants for maintaining support of a body lumen.
An important use of stents is found in situations where part of the vessel wall or stenotic plaque blocks or occludes fluid flow in the vessel. Often, a balloon catheter is utilized in a percutaneous transluminal coronary angioplasty procedure to enlarge the occluded portion of the vessel. However, the dilation of the occlusion can cause fissuring of atherosclerotic plaque and damage to the endothelium and underlying smooth muscle cell layer, potentially leading to immediate problems from flap formation or perforations in the vessel wall, as well as long-term problems with restenosis of the dilated vessel. Implantation of stents can provide support for such problems and prevent re-closure of the vessel or provide patch repair for a perforated vessel. Further, the stent may overcome the tendency of diseased vessel walls to collapse, thereby maintaining a more normal flow of blood through that vessel.
Significant difficulties have been encountered with all prior art stents. Each has its percentage of thrombosis, restenosis and tissue in-growth, as well as various design-specific disadvantages.
Examples of prior developed stents have been described by Balcon et al., xe2x80x9cRecommendations on Stent Manufacture, Implantation and Utilization,xe2x80x9d European Heart Journal (1997), vol. 18, pages 1536-1547, and Phillips, et al., xe2x80x9cThe Stenter""s Notebook,xe2x80x9d Physician""s Press (1998), Birmingham, Mich. The first stent used clinically was the self-expanding xe2x80x9cWallstentxe2x80x9d which comprised a metallic mesh in the form of a Chinese fingercuff. This design concept serves as the basis for many stents used today. These stents were cut from elongated tubes of wire braid and, accordingly, had the disadvantage that metal prongs from the cutting process remained at the longitudinal ends thereof. A second disadvantage is the inherent rigidity of the cobalt based alloy with a platinum core used to form the stent, which together with the terminal prongs, makes navigation of the blood vessels to the locus of the lesion difficult as well as risky from the standpoint of injury to healthy tissue along the passage to the target vessel. Another disadvantage is that the continuous stresses from blood flow and cardiac muscle activity create significant risks of thrombosis and damage to the vessel walls adjacent to the lesion, leading to restenosis. A major disadvantage of these types of stents is that their radial expansion is associated with significant shortening in their length, resulting in unpredictable longitudinal coverage when fully deployed.
Among subsequent designs, some of the most popular have been the Palmaz-Schatz slotted tube stents. Originally, the Palmaz-Schatz stents consisted of slotted stainless steel tubes comprising separate segments connected with articulations. Later designs incorporated spiral articulation for improved flexibility. These stents are delivered to the affected area by means of a balloon catheter, and are then expanded to the proper size. The disadvantage of the Palmaz-Schatz designs and similar variations is that they exhibit moderate longitudinal shortening upon expansion, with some decrease in diameter, or recoil, after deployment. Furthermore, the expanded metal mesh is associated with relatively jagged terminal prongs, which increase the risk of thrombosis and/or restenosis. This design is considered current state of the art, even though their thickness is 0.004 to 0.006 inches.
Another type of stent involves a tube formed of a single strand of tantalum wire, wound in a sinusoidal helix; these are known as coil stents. They exhibit increased flexibility compared to the Palnaz-Schatz stents. However, they have the disadvantage of not providing sufficient scaffolding support for many applications, including calcified or bulky vascular lesions. Further, the coil stents also exhibit recoil after radial expansion.
One stent design described by Fordenbacher, employs a plurality of elongated parallel stent components, each having a longitudinal backbone with a plurality of opposing circumferential elements or fingers. The circumferential elements from one stent component weave into paired slots in the longitudinal backbone of an adjacent stent component. By incorporating locking means within the slotted articulation, the Fordenbacher stent may minimize recoil after radial expansion. In addition, sufficient numbers of circumferential elements in the Fordenbacher stent may provide adequate scaffolding. Unfortunately, the free ends of the circumferential elements, protruding through the paired slots, may pose significant risks of thrombosis and/or restenosis. Moreover, this stent design would tend to be rather inflexible as a result of the plurality of longitudinal backbones.
Some stents employ xe2x80x9cjelly rollxe2x80x9d designs, wherein a sheet is rolled upon itself with a high degree of overlap in the collapsed state and a decreasing overlap as the stent unrolls to an expanded state. Examples of such designs are described in U.S. Pat. No. 5,421,955 to Lau, U.S. Pat. Nos. 5,441,515 and 5,618,299 to Khosravi, and U.S. Pat. No. 5,443,500 to Sigwart. The disadvantage of these designs is that they tend to exhibit very poor longitudinal flexibility. In a modified design that exhibits improved longitudinal flexibility, multiple short rolls are coupled longitudinally. See e.g., U.S. Pat. No. 5,649,977 to Campbell and U.S. Pat. Nos. 5,643,314 and 5,735,872 to Carpenter. However, these coupled rolls lack vessel support between adjacent rolls.
Another form of metal stent is a heat expandable device using Nitinol or a tin-coated, heat expandable coil. This type of stent is delivered to the affected area on a catheter capable of receiving heated fluids. Once properly situated, heated saline is passed through the portion of the catheter on which the stent is located, causing the stent to expand. The disadvantages associated with this stent design are numerous. Difficulties that have been encountered with this device include difficulty in obtaining reliable expansion, and difficulties in maintaining the stent in its expanded state.
Self-expanding stents are also available. These are delivered while restrained within a sleeve (or other restraining mechanism), that when removed allows the stent to expand. Self-expanding stents are problematic in that exact sizing, within 0.1 to 0.2 mm expanded diameter, is necessary to adequately reduce restenosis. However, self-expanding stents are currently available only in 0.5 mm increments. Thus, greater selection and adaptability in expanded size is needed.
In summary, there remains a need for an improved stent: one that has smoother marginal edges, to minimize restenosis; one that is small enough and flexible enough when collapsed to permit uncomplicated delivery to the affected area; one that is sufficiently flexible upon deployment to conform to the shape of the affected body lumen; one that expands uniformly to a desired diameter, without change in length; one that maintains the expanded size, without significant recoil; one that has sufficient scaffolding to provide a clear through-lumen; one that employs a thinner-walled design, which can be made smaller and more flexible to reach smaller diameter vessels; and one that has a thinner-walled design to permit faster endothelialization or covering of the stent with vessel lining, which in turn minimizes the risk of thrombosis from exposed stent materials.
The present invention relates to an expandable intraluminal stent, comprising a tubular member with a clear through-lumen. The tubular member has proximal and distal ends and a longitudinal length defined therebetween, and a circumference, and a diameter which is adjustable between at least a first collapsed diameter and at least a second expanded diameter. In a preferred mode, the longitudinal length remains substantially unchanged when the tubular member is adjusted between the first collapsed diameter and the second expanded diameter. The tubular member includes at least one module comprising a series of sliding and locking radial elements, wherein each radial element defines a portion of the circumference of the tubular member and where no radial element overlaps with itself in either the first collapsed diameter or the second expanded diameter.
In one aspect, each radial element may comprise at least one elongated rib disposed between first and second end portions. Preferably, the radial elements that comprise a module alternate between radial elements having an odd number of elongated ribs and radial elements having an even number of elongated ribs. In one preferred mode, the radial elements alternate between radial elements having one elongated rib and radial elements having two elongated ribs.
The stent also includes at least one articulating mechanism comprising a tab and at least one stop. The articulating mechanism permits one-way sliding of the radial elements from the first collapsed diameter to the second expanded diameter, but inhibits radial recoil from the second expanded diameter.
In variations to the stent, the tubular member may comprise at least two modules which are coupled to one another by at least one linkage element. In one variation, the tubular member may further comprise a frame element that surrounds at least one radial element in each module. In stents in which the tubular member comprises at least two modules, such frame elements from adjacent modules may be coupled. The coupling may include a linkage element extending between the frame elements. In addition or in the alternative, the frame elements from adjacent modules may be coupled by interlinking of the frame elements. In another aspect, the intermodular coupling may be degradable allowing for the independent modules to adapt to the vessel curvature.
In another variation to the stent of the present invention, any amount of overlap among the radial elements within in a module remains constant as the tubular member is adjusted from the first collapsed diameter to the second expanded diameter. This amount of overlap is preferably less than about 15%.
The radial recoil of the tubular member in accordance with one preferred embodiment is less than about 5%. The stiffness of the stent is preferably less than about 0.1 Newtons force/millimeter deflection. The tubular member also preferably provides a surface area coverage of greater than about 20%.
In accordance with another variation of the present stent, the tubular member is at least partially radiopaque. The radial elements may be made substantially from a material which is work hardened to between about 80% and 95%. In one preferred variation, the radial elements in the expandable intraluminal stent are made from a material selected from the group consisting of a polymer, a metal, a ceramic, and combinations thereof. In one mode, the material may be degradable.
In another mode of the invention, the material may also include a bioactive agent. The material is preferable adapted to deliver an amount of the bioactive agent which is sufficient to inhibit restenosis at the site of stent deployment. In one variation, the radial elements are adapted to release the bioactive agent during stent deployment when the tubular member is adjusted from the first collapsed diameter to the second expanded diameter. The bioactive agent(s) is preferably selected from the group consisting of antiplatelet agents, antithrombin agents, antiproliferative agents, and antiinflammatory agents.
In another variation, the tubular member further comprises a sheath, such as for example in a vessel graft.
In one aspect, the expandable intraluminal stent comprises at least two modules, wherein the expanded diameters of the first and second modules are different.
The articulating mechanism(s) of the present invention which allow the stent to expand but inhibit stent recoil, may comprise a slot and a tab on one radial element and at least one stop on an adjacent radial element which is slideably engaged in the slot, wherein the tab is adapted to engage the at least one stop. The articulating mechanism(s) may also include an expansion resistor on the slideably engaged radial element, wherein the expansion resistor resists passing through the slot during expansion until further force is applied, such that the radial elements in the module expand in a substantially uniform manner. In another variation, the articulating mechanism may include a release, such that actuation of the release permits sliding of the radial elements from the second expanded diameter back to the first collapsed diameter for possible removal of the stent. In another variation, the stent may comprise a floating coupling element having an articulating mechanism.
In another variation, the expandable intraluminal stent comprises a tubular member with a clear through-lumen and a diameter which is adjustable between at least a first collapsed diameter and at least a second expanded diameter. The tubular member comprises a series of sliding and locking radial elements made from a degradable material, wherein each radial element in the series defines a portion of the circumference of the tubular member and wherein no radial element overlaps itself. This stent also has at least one articulating mechanism that permits one-way sliding of the radial elements from the first collapsed diameter to the second expanded diameter, but inhibits radial recoil from the second expanded diameter. The degradable material may be selected from the group consisting of polyarylates (L-tyrosine-derived), free acid polyarylates, polycarbonates (L-tyrosine-derived), poly(ester-amides), poly(propylene fumarate-co-ethylene glycol) copolymer, polyanhydride esters, polyanhydrides, polyorthoesters, and silk-elastin polymers, calcium phosphate, magnesium alloys or blends thereof.
In a variation to the degradable stent, the degradable polymer may further comprise at least one bioactive agent, which is released as the material degrades. The at least one bioactive agent may be selected from the group consisting of antiplatelet agents, antithrombin agents, antiproliferative agents and antiinflammatory agents.
In another variation, the stent material may be fiber-reinforced. The reinforcing material may be a degradable material such as calcium phosphate (e.g., BIOGLASS). Alternatively, the fibers may be fiberglass, graphite, or other non-degradable material.
In another mode, the stent of the present invention comprises a tubular member having a wall and a clear through-lumen. The tubular member comprises a series of sliding and locking radial elements which do not overlap with themselves. The radial elements further comprise a ratcheting mechanism that permits own-way sliding of the radial elements from a first collapsed diameter to a second expanded diameter. The tubular member in this embodiment has a stiffness of less than about 0.1 Newtons force/millimeter deflection, and the wall of the tubular member has a thickness of less than about 0.005 inches.